A gravitational constant that is the same 
  everywhere in the universe and is incompatible 
  with a fifth force.

As a constant, that means that its invariance is significant, in the same way the speed of light is significant. There is some externality pegging the phenomena we notice, at the value we observe. Perhaps some sort of Planck-level absolute fact, which is irreducible in the same way that the concept of color doesn't exist very much beneath 400 nanometers (violet/near-ultraviolet), since color is only an abstraction of our eyeballs and the language we use to describe our sensations.

Glancing at some of the other things you've said in the thread, you seem to be convinced that some waves, like sound waves, are somehow illusory. Let me assure you, there is nothing illusory about wave phenomena, both in general and with gravitational waves in particular. Anything that produces wave-like phenomena can be said to radiate emissions, to paraphrase, and in many cases, there is nothing really more "fundamental" than the wave.

As for the concept of color, I suppose it depends on what you mean by color. Color as defined by the wavelength/frequency of light is perfectly well defined outside of the visible spectrum. As a mental concept, I don't see why it is "irreducible." I suppose you are attempting to say that the universal speed of light is somehow fundamental... but in the same way that a "redder" red is impossible? This is a dubious analogy. It confuses the limitations of the mind with fundamental physics.

You might be in love with the idea of describing an equation that frames the gradient of distribution, and the nature of it's propagation through a medium, but the sound wave is the manner in which the gaseous molecular constituents of the air are set in motion relative to one another. They get closer, they move apart, the changes occur at different places in the medium, at different times, and do so at a certain velocity, in sequence as interactions are forced upon the medium.

Indeed, the reason sound waves travel at the speeds we observe, is because that's how fast the very molecules themselves, comprising the air, are moving at the temperature and pressure of the environment.

Meanwhile, what color is a beam with a wavelength of one nanometer? Would you characterize the color as "soft x-ray"?

Further, not all waves require a medium. Light and gravitational waves are prominent examples. There is nothing to reduce these phenomena to, except for the fields themselves, whose form is dictated by... a wavelike solution.

While we are at it, let me disabuse you of your explanation of the sound speed. Turns out that the sound speed is a thermodynamic quantity; it is the speed at which small wavelike perturbations propagate. To properly derive the sound speed, one must linearize the Euler equations, and then adopt a thermodynamic equation of state, from which the sound speed is derived. It is, emphatically, not the speed at which molecules move.

Finally, it is obvious that you did not understand what I was driving at with respect to color. I agree wholeheartedly that we have no true color, within our minds, with which to perceive, say, soft x-rays. But this issue has to do with our own neurobiology, not fundamental physics. Comparing the two is what is problematic. There is little reason to suspect that our mental limitations have anything to do with anything but evolutionary necessity. Such limitations are categorically different than, say, the speed of light.

But then gravity changes would be felt instantaneously rather than propagating at the speed of light.

From Wikipedia:

  Light spots and shadows

https://en.wikipedia.org/wiki/Faster-than-light

Of course, the shadow must already be cast, in a continuous stream of uniquitous photons, which all need to be arriving upon the reflective surface at the speed of light, before the shadow can transit the surface within the area of effect.

At best, only the destruction of matter can move at the speed of light, so a reduction of gravity may propagate, as fast as the dispersal of the dematerialization and subsequent radiation.

Accumulation of mass as a knock-on pile-up of massive particles would need to be studied, to assess how quickly the shadow of newly created matter can induce its respective gravitational phenomenon. Figure star nurseries and supernovae events (and similar generative or cataclysmic reactions) would be the place to look. LIGO studies try to do exactly that, but the investigators seem to relish conflating an interpretation that favors signal transmission, and thus "particle/wave duality" because math.

Gravity-as-a-wave, meanwhile would be sort of like an illusory sound-as-a-wave being a byproduct of kinetic energy in a material aggregate serving as a bystander medium of propagation. Sound is not it's own "force" although we can readily recognize the phenomenon as a principle that behaves with similar attributes, parameters and effects as observed elsewhere, in other fundamental systems.

That LIGO instruments can detect gravitational influence in one part of the apparatus, before the other part of the apparatus registers the phenomenon, is really not unlike saying dominos demonstrate gravitational waves, because toppling the first domino does not topple the last domino instantaneously.

That photons can be influenced by a side-effect of material accumulation, bending or altering their path of travel probably says something about the photon or the environment being travelled through, more than it does about space and time.

But the environment they travel through is space and time.

Semantic in the sense that the word "environment" is a loaded term.

I'll offer this much: photons are particles. As particles, we know them to be part-time resident constituents of massive objects. That massive objects are representative of large quantities of energy trapped and oscillating in bounded standing waveforms.

We know that the empty vacuum of space really is mostly void, but also that a lot of photons (particles) are still traversing the void quite a bit.

So, space/time, while an empty void, where phenomena transit or conduct themselves theough said empty medium, is still an environment with varying concentrations of activity.

If an neutron manages to be ejected from a supernova, and is directed toward our solar syatem, and is influenced to fall toward the sun, but, miraculously passes straight through the sun, without colliding with any other particles, only to travel onward toward the center of the galaxy, landing in a supermassive black hole some millennia from now, what would you say about the environment this neutron experienced as it fortuitously passed through the center of the sun, without experiencing any collisions?

Would to rob it of all the other qualities, and just label it space and time? Or would you refer to it as an environment, with activity transpiring, in addition to be also being a realm of space and time?

Photons are also waves in the EM field.

> As particles, we know them to be part-time resident constituents of massive objects.

I don't think you can say we know that without explain what that means. In no part of physics do we discuss part-time resident constituents.

Your comments use a lot of terms that seem to be of your own creation. Unfortunately, these terms make it hard for your points to be understood because they lack the shared naming conventions that the field of physics uses to discuss these phenomena. Standard naming conventions, both in programming and physics, are enormously important in conveying ideas between people.

https://en.wikipedia.org/wiki/Photoelectric_effect

The whole point of the paper is studying the effect of gravity on gravitational self-energy, i.e. NOT matter. You'll find no "matter term" in Einstein's equations.

In the observable universe (regardless of theories and expressions penned on paper) gravitational forces are always induced by massive material objects, including the remnant artifacts of deceased stars. To suggest otherwise is to walrus the conversation.

Being a theoretical hypothesis, formed to explain observations, it remains a side-effect of accounting and inference, and has not been located directly.

Show me why you believe it is a thing. Show me where you have found it. Tell me why it isn't actually matter.